SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of securities and other investments owned and securities sold not yet purchased at fair value (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Securities and other investments owned:
Total securities and other investments owned	$ 1,317,101	$ 1,532,095
Securities sold not yet purchased:
Total securities sold not yet purchased	7,498	28,623
Corporate bonds
Securities and other investments owned:
Total securities and other investments owned	10,508	7,632
Securities sold not yet purchased:
Total securities sold not yet purchased	6,046	6,327
Other fixed income securities
Securities and other investments owned:
Total securities and other investments owned	3,681	2,606
Securities sold not yet purchased:
Total securities sold not yet purchased	334	1,994
Partnership interests and other
Securities and other investments owned:
Total securities and other investments owned	74,222	77,383
Equity securities
Securities and other investments owned:
Total securities and other investments owned	1,228,690	1,444,474
Securities sold not yet purchased:
Total securities sold not yet purchased	$ 1,118	$ 20,302